I'm afraid that not everybody shares that view.
The economic requirements are inevitably the bottom line, but there are other factors. The Vuelta has more to think about than TV ratings. It's not the Tour of California. This race has history and tradition to uphold. It has to sell the race to Spaniards in order to bring back the support level that allows them the best chance to sell the race to the world.
There's also short- and long-term. Bringing Radioshack in, and then everything kicks off while they're racing there will not put all eyes on the Vuelta. It will put all eyes on Lance, who won't even be there. That will have no benefit for the Vuelta, in fact it will be bad because the chances are either Radioshack will be forced to withdraw, damaging the race, or they will continue to compete under a cloud.